We propose to use repetitive sequences in search of genes that are involved in the pathology associated with DS on other diseases affected by chromosome 21 and also to explore the regulatory function of repetitive sequences in the expression of these and other genes in the brain. The following experiments are planned: 1. Use of cloned repetitive sequences as genetic markers to identify and isolate abnormal DNA fragments along chromosomal 21 defining deletions, translocations and other rearrangements observed in Down syndrome and other patients. 2. Characterization of DNA fragments isolated in specific aim #1. Sequencing analysis will reveal any open reading frames and whether gene is amplified. Chromosomal mapping is carried out to locate such sequences on chromosome 21. 3. Expression of gene sequences at the RNA and protein level. We shall use gene sequences obtained in specific aim #1 as well as existing cDNA clones from fetal brain cDNA library mapped to chromosome 21. In situ hybridization to RNA in tissue section of fetal brain from trisomy 16 mouse as compared to that of normal mouse will identify gene sequences that reveal preferential hybridization to one or the other. Such sequences and others will be subcloned into expression vector and the protein product from such recombinants will be injected into mice for antibody production. Immunofluorescence of such antibody which binds to tissue section of fetal brain of trisomy 16 mouse or that from normal mouse will reveal quantitatively or qualitatively whether these gene sequences are expressed differently in the brain of a fetal trisomy 16 mouse. 4. Study of repetitive sequences that are human specific and are present on chromosome 21. Repetitive sequences specific to the human species will be isolated from chromosome 21 genomic library by differential hybridization. Their regulatory role will be explored especially in their control of expression of genes In the brain.